Surge protection circuit

ABSTRACT

A regulated power supply is protected from input potential surges by a transistor and Zener diode input network which shuts down the regulated supply whenever the input potential exceeds a predetermined maximum level. When an input potential surge occurs, current flows through a Zener diode and transistor series circuit connected across the input terminals, and a shutdown current flows from the transistor into a shutdown node within the power supply.

United 7 States Patent Inventor Appl. No.

Filed Patented Assignee Ivan R. Damon Tlnley Park, 111. 799,678

Feb. 17, 1969 May 18, 1971 SCM Corporation New York, N.Y.

suncn PROTECTION cmcurr 16 Claims, 2 Drawing Figs.

U.S. Cl

317/31, 317/33, 317/50, 323/9 Int. Cl H02h 3/22 Field of Search 317/50,22, 31, 33 (VR); 323/9, 22 (T), (inquired), 4

[56] References Cited UNITED STATES PATENTS 3,048,718 8/1962 Starzec etal. 317/33X 3,049,632 8/1962 Staples 3 l 7/33X Primary ExaminerJames D.Trarnmell- Attorney- Mason, Kolehmainen, Rathburn & Wyss ABSTRACT: Aregulated power supply is protected from input potential surges by atransistor and Zener diode input network which shuts down the regulatedsupply whenever the input potential exceeds a predetermined maximumlevel; When an input potential surge occurs, current flows through aZener diode and transistor series circuit connected across the inputterminals, and a shutdown current flows from the transistor into ashutdown node within the power supply.

Patented May 18, 1971 3,519,039

IVAN l2. DAMON WZMW W 1 SURGE PROTECTION CIRCUIT The present inventionrelates to regulated power supplies, and more particularly to protectivecircuits for shutting down a regulated power supply to protect itagainst input potential surges.

Transistorized regulated power supplies are widely used to supplyconstant currents or voltages to electronic circuits. A typicalregulated supply generally comprises a series regulating elementconnected in series with a load across a source of unregulatedpotential, a Zener diode potential reference element, and circuitry forcomparing the Zener reference potential with either the output voltageor current and for varying the conduction of the series regulatingelement so as to maintain the output voltage or current constant both inthe face of input potential fluctuations and load fluctuations.

A major problem in the design of such supplies is conducting heat awayfrom the series regulating element. Generally this element must bedesigned to dissipate power quantities comparable in magnitude to thepower dissipated by the entire load. The heat dissipation problembecomes especially critical in compact, portable devices wherein thespace available for heat sinks and heat-radiating devices is at apremium. Not only is it necessary to protect the series regulatingelement from the destructive effects of heat generated under normaloperating conditions, but it is also necessary to provide protectionagainst heat generated during current overload or input potentialtransients. In the case of a constant voltage power supply, currentoverload protection is usually built into the supply, so no additionalheat sinking is required. It has been the customary practice to provideno electronic protection against input potential transients, however.The customary practice has been to choose a heat sink having sufficientcapacity to provide protection against the worst possible case ofexcessive input potential. Since input potential transients often arehalf again greater than the normal system input potential, it is oftennecessary to provide twice the heat dissipation capacity required tohandle normal circuit requirements merely as protection againstoccasional input voltage overload.

A primary object of the present invention is therefore before theconstruction of a regulated power supply having a heat dissipationelement that is as small and as compact as possible, and which isdesigned to dissipate only the heat encountered under normal operatingconditions.

Another object of the present invention is the construction of aregulated power supply that includes a surge protector which terminatesthe output current during periods when the input potential exceeds apredetermined maximum level, and which automatically returns the supplyto a regulating state when the input potential drops below thepredetermined maximum level.

A further object of the present invention is the provision of such asupply wherein the termination of the output current occurs atelectronic speeds, so that the power supply and the load are protectedagainst transitory input potential surges.

In accordance with these and many other objects, the present inventioncomprises a regulated power supply which includes a circuit thatautomatically shuts down the supply when the supply input potentialexceeds a predetermined maximum level. This regulating circuit includesa first Zener diode connected in series with the base-emitter junctionof a transistor across the source of input potential. The Zener diode ischosen to have a Zener potential which is approximately equal to thepotential at which it is desired to shut down the supply.Current-limiting resistors can be included in series with the Zenerdiode, and in series with the emitter lead of the transistor. A shutdowncurrent then flows from the collector of the transistor whenever theinput potential exceeds the Zener potential of the Zener diode.

This shutdown current is used to turn off the regulated power supply. Ina preferred embodiment of the invention, the regulated power supplyincludes a Zener diode reference element that is supplied with aconstant current from a conventional Zener diode and transistor constantcurrent source. In

the case of such a supply, the shutdown current is used to bias off thetransistor in the constant current source and thereby terminate theconstant current. This causes the voltage across the Zener referenceelement to collapse and thereby causes the supply to shut down. Themagnitude of the shutdown current is limited by a second Zener diodeconnected between the base of the shutdown current transistor and theinput potential supply terminal connected to the emitter of thistransistor. Heat dissipation in the shutdown current transistor can belimited by inserting a resistor in the path of the shutdown current.

The invention, together with other objects and advantages thereof, willbest be understood from considering the following detailed descriptionin conjunction with the drawings wherein:

FIG. 1 is a schematic diagram of a constant current regulated powersupply including a surge protection circuit designed in accordance withthe present invention; and

FIG. 2 is a schematic diagram of a constant voltage power supply thatincludes a surge protection circuit designed in accordance with thepresent invention.

Referring now to the drawings, FIG. 1 shows a constant current regulatedpower supply including a surge protection circuit designed in accordancewith the present invention, and indicated generally as 10. Input voltagefor the power supply 10 is supplied from positive and negative supplynodes respectively designated 14 and 16. The power supply 10 draws aconstant current from the node 14, passes it through a load 12, andreturns it to the node 16. In accordance with the teachings of thepresent invention, a surge protection circuit 52 supplies a shutdowncurrent to a shutdown node 50 within the power supply 10 when thepotential between the supply nodes 14 and I6 exceeds a predeterminedmaximum allowable value. The potential at which the protection circuit52 shuts down the power supply 10 is determined by a Zener diode 54. TheZener diode 54 is connected in series with the base-emitter junction ofa transmitter 56 directly across the supply nodes 14 and 16. A resistor58 connected in series with the Zener diode 54 limits current drawn bythe diode 54 to a safe value, and another resistor 60 connected betweenthe emitter 62 of the transistor 56 and the supply node 14 limitscurrent flow in the transistor 56.

Normally the voltage across the supply nodes 14 and 16 is of a lowermagnitude than the Zener voltage of the Zener diode 54, and the Zenerdiode 54 remains nonconductive. No current flows into the base lead 64of the transistor 56, and therefore no current flows out of thecollector lead 66 of the transistor 56. During a potential surge, thevoltage across the nodes 14 and 16 exceeds the Zener voltage of thediode 54, and current flows from the node 14, through elements 60, 56,54, and 58 to the node 16. This current is amplified by the transistor56 and appears as a shutdown current which flows out the collector lead66 of the transistor 56 and into the shutdown node 50 of the regulatedpower supply 10, in a manner that will be explained in detail below.

To prevent the base-emitter voltage of transistor 26 from.

exceeding its safe operating limits and to limit the current intransistor 56, a Zener diode 68 is connected between the supply node 14and the base lead 64 of the transistor 56. This second Zener diode 68causes the three elements 56, 60, and 68 to function as a constantcurrent source, and thereby limits the magnitude of the shutdown currentto a safe value approximately equal to the Zener voltage of the diode 68divided by the resistance of the resistor 60.

In the case of a prolonged surge, it is possible that current flowthrough the transistor 56 can cause excessive heating. Therefore aheat-dissipating resistor 70 is connected between the collector 66 ofthe transistor 56 and the shutdown node 50. The resistor 70 alsoprovides some protection to the transistors 26 and 56 in case the Zenerdiode 68 should become open circuited.

Referring now to FIG. 2, there is shown a constant voltage regulatedpower supply designated generally as 110. The

supply 110 draws current from a positive supply node 114, passes it to aload 112, and returns it to a negative supply node 116. The supply 110is designed to maintain a constant voltage across the load 1.12. Inaccordance with the teachings of the present invention, a surgeprotection circuit 152 is arranged to turn off the power supply 110 ifthe voltage between the nodes 114 and 116 exceeds a predetermined safemaximum value. The surge protection circuit 152 supplies a shutdowncurrent to a shutdown node 150 within the power supply 110 whenever aninput voltage surge occurs. As will be explained below, this currentcauses the supply 110 to cease supplying current to the load 112.

Operation of the surge protection circuit 152 is identical to that ofthe circuit 52 shown in H6. 1 except that the circuit 152 is designed tosupply a shutdown current of the opposite polarity from that supplied bythe circuit 52. The elements of the circuit 152 are numbered tocorrespond with those of the circuit 52. The Zener diode 154 connects tothe positive node 114, and the emitter 162 of the transistor 156connects to the negative supply node 116 through the resistor 160. TheZener diode 154 is oriented in the same way as the Zener diode 54, withits cathode lead pointing towards the positive supply node, but thetransistor 156 is complementary to the transistor 56-that is, thetransistor 156 is an NPN transistor, while the transistor 56 is a PNPtransistor. ln all other respects, the circuits 52 and 152 areessentially identical. When the potential between the supply nodes 114and 116 exceeds the Zener voltage of the Zener diode 168, a shutdowncurrent flows into the shutdown node 150 and causes the power supply 110to cut off the flow of current to the load 112. When the potentialbetween the supply nodes 114 and 116 once again returns to a safe value,the shutdown current is terminated and the power supply 110 returns toits normal mode of operation.

Referring again to FIG. 1, the power supply is a constant current powersupply, and it feeds a constant current to the load 12. Seriallyconnected Zener diodes 18 and 20 limit the maximum output potentialwhich can be applied to the load 12 and diodes 19 and 21 compensate fortemperature drift in Zener diodes 18 and 20. A transistor 22 is theseries regulating element, and a Zener diode 24 provides a referencepotential. A diode 25 compensates for temperature drift. The Zener diode24 is supplied with a constant current by a transistor 26, and resistors36 and 40, and a Zener diode 38, which together comprise a constantcurrent source. The Zener diode 24 thus presents a constant outputpotential to a transistor 28. The circuit 10 maintains the outputcurrent at a constant level by maintaining a constant voltage across avariable resistor which is in series with the load 12 and the seriesregulating transistor 22. The transistors 28 and 22 act as emitterfollower amplifiers. They maintain a constant voltage across theresistor 30 that is approximately equal to the voltage across thereference Zener diode 24. A resistor 32 in the collector circuit of thetransistor 28 limits the maximum current which can flow through the baselead of the transistor 22, and thereby provides some protection to boththe transistors 22 and 28 in case the Zener diode 24 becomes opencircuited. This resistor also reduces the power dissipation in thetransistor 28. A resistor 34 provides a low-impedance path between theemitter and the base of the series regulating transistor 22, therebygreatly reducing the amount of leakage output current which flows whenthe supply 10 is shut down. This resistor 34 greatly decreases thechances of the transistor 22 entering a destructive thermal runawaycondition. The reference voltage supplied by the Zener diode 38 issustained by current flowing from the positive node 14 through aresistor 40.

When the constant current regulated power supply 10 is shut down by aninput voltage surge, a shutdown current from the surge protectioncircuit is fed into the node 50. This current builds up a voltage acrossthe resistor 36 which is greater than the Zener voltage of the Zenerdiode 38, and therefore biases the constant current source transistor 26into a nonconducting state. This deprives the Zener diode 24 of itssustaining current, and causes the Zener diode 24 to becomenonconductive. This also deprives the transistor 28 of current suffcientto maintain conduction in the transistor 22. The transistor 22 thenbecomes nonconductive, and remains so until the flow of shutdown currentinto the node 50 from the surge protection circuit 52 is once againterminated. When this happens, the circuit 10 returns to its normal modeof operation.

Referring now to P16. 2, the power supply is seen to be of conventionaldesign. It includes a series regulating transistor 122 which controlsthe flow of current to the load 112. A reference voltage is maintainedacross four serially connected Zener diodes 124 by current flow from aconstant current source comprising resistors 136 and 140, a Zener diode138, and a transistor 126. A transistor 128 and the series regulatingelement transistor 122 function as a Darlington amplifier and maintain avoltage across the load 112 that is approximately equal to the voltageacross the Zener diodes 124. The diodes 124 may be replaced by aconventional power supply feedback amplifier, if desired. Resistor 134functions to limit leakage current in the transistor 122 when the powersupply 110 is shut down, thereby preventing thermal runaway of thetransistor 122. Series resistor measures current flowing to the load112. When the load current becomes excessive, the voltage developedacross the resistor 130 causes a transistor 118 to conduct and to shortcircuit the constant current from the transistor 126 to the emitterterminal 120 of the transistor 118. This causes the transistors 128 and122 to cease conduction when the load 112 draws excessive current,thereby limiting the maximum possible output current to a safe value.

The shutdown node is the junction point between the resistor 136 and theemitter of the transistor 126. When the shutdown current is fed intothis node, the potential across the resistor 136 rises to a value inexcess of the Zener voltage of the Zener diode 138, and causes thetransistor 126 to become nonconductive. This terminates the flow ofcurrent to the base of the transistor 122. A shutdown current thus stopsthe flow of current to the load 112. When the shutdown current isterminated, the transistor 126 once again supplies current to the Zenerdiodes 124, and the power supply 110 immediately resumes its normal modeof operation.

A capacitor 127 is connected between the emitter and base elements ofthe transistor 127 to suppress spurious high frequency oscillations.

The constant current power supply 10 and the constant voltage powersupply 110 have been selected as representative of the various types ofpower supplies with which the present invention can be used. Powersupplies differ in design from one another, and it may be necessary tomodify the surge protection circuit in order to facilitate itsincorporation into a particular power supply. Some supplies do not havea convenient node into which a shutdown current can be fed, and

' may therefore require additional elements for converting the shutdowncurrent into a suitable signal for shutting down the supply. In powersupplies not utilizing a constant current source to supply a Zenervoltage sustaining current, the shutdown current can be fed to atransistor that shorts out the Zener diode when it receives current, forexample.

Although the present invention has been described with reference to anillustrative embodiment thereof it should be understood that numerousmodifications and changes will readily occur to those skilled in the artand it is therefore intended by the appended claims to cover all suchmodifications and changes as fall within the true spirit and scope ofthe invention.

lclaim:

l. A regulated power supply of the type in which a reference potentialsource is used as a standard against which the output voltage or currentis compared, wherein the improvement comprises a surge protectioncircuit for turning off the supply when the potential input to thesupply exceeds a predetermined potential level including a device thatis normally nonconductive and that becomes conductive when subjected toa I potential approximately equalin magnitude to said predeterminedlevel, series circuit means connecting said device across the potentialinput to the supply, and shutdown means connected to the referencepotential source for greatly reducing the magnitude of the referencepotential upon sensing a flow of current through said device.

2. A surge protection circuit for turning off a regulated power supplywhen the potential input to the supply exceeds a predetermined potentiallevel, said circuit comprising:

a device that is, normally nonconductive, and that becomes conductivewhen subjected to a potential approximately equal in level to saidpredetermined potential level;

series circuit means connecting said device across the potential inputto the supply;

signal amplification means included in said series circuit means forproducing a shutdown current in response to current flow through saiddevice; and

shutdown 'means within said supply for turning off said supply inresponse to said shutdown current.

3. A surge protection circuit in accordance with claim 2 wherein thedevice is a reverse-biased Zener diode having a Zener potential levelapproximately equal to said predetermined potential level.

4. A surge protection circuit in accordance with claim 2 wherein theseries circuit means comprises the amplification means and atleast oneresistive element connected in series with said device to 'form a seriescircuit, and wherein said series circuit is connected across thepotential input to the supply.

5. A surge protection circuit in accordance with claim 2 wherein theregulated power supply includes a potential reference device that issupplied with a current, and wherein the shutdown means comprisesmeans'for terminating this current whenever the shutdown current isflowing.

6. A surge protection circuit in accordance with claim 2 wherein theregulated power supply includes a reference potential, and wherein theshutdown means comprisesmeans for greatly reducing the magnitude of thisreference potential.

7. A surge protection circuit in accordance with claim 2 wherein theregulated power supply includes a series regulating element, and whereinthe shutdown means comprises means for halting the flow of currentthrough this series regulating element.

8. A surge protection circuit for turning off a regulated power supplywhen the potential input to the supply exceeds a predetermined potentiallevel, said power supply including a constant current source having ashutdown node, said circuit comprising: i

a device that is nonnally nonconductive, and that becomes conductivewhen'subjected to a potential approximately equal in level to saidpredetermined potential level;

- series circuit means connecting said device across the potential inputto said supply; and

amplification means for amplifying current flow through said device intoa shutdown current which is fedinto the shutdown node within theconstant current source.

9. A surge protection circuit in accordance with claim 8 wherein theconstant current source within the supply feeds current to a Zener diodereference potential circuit within the supply.

10. A surge protection circuit for generating a shutdown current inresponse to the potential level of a potential source exceeding apredetermined maximum, said circuit comprising:

a device that is normally nonconductive, and that becomes conductivewhen subjected to a potential approximately equal in level to saidpredetennined level; and

a series circuit connected across said potential source, and

including:

said device,

a first current-limiting resistor connected directly to said device toform a series subcircuit,

a transistor having emitter, base, and collector electrodes, and havingits base electrode connected to said series subcircuit, and

a second current limiting resistor connected to the emitter electrode ofsaid transistor; whereby a shutdown current flows from the collectorelectrode of said transistor when the potential level of the potentialsource exceeds the predetermined maximum.

11. A surge protection circuit in accordance with claim 10 and furtherincluding a second Zener diode connecting the emitter electrode of saidtransistor to the end of said second current-limiting resistor that doesnot connect to the emitter of said transistor.

12. A regulated power supply for regulating the flow of power from apower source to a load, said supply comprising:

a series regulating element;

a first series circuit connected across said power source, and

including said series regulating element and the load;

a reference potential source;

feedback circuit means for comparing the potential across said load withthe potential across said reference potential source, and for adjustingthe conduction of said series regulating element to maintain the ratioof these two potentials substantially constant;

a device that is normally nonconductive and that becomes conductive whensubjected to a potential which exceeds a predetermined potential level;

a second series circuit connected across said power source and includingsaid device; and

shutdown means responsive to current flow through said device forgreatly reducing the potential generated by the reference potentialsource, whereby the power supply ceases to pass current to the load whenthe potential of the power source exceeds a level.

13. A regulated power supply in accordance with claim '12 which alsoincludes a constant source supplying a constant current to theregulating element, and wherein the shutdown means stops the flow ofcurrent from the constant current source.

14. A surge protection circuit for turning off a regulated power supplywhen the potential input to the supply exceeds a predetermined potentiallevel, said circuit comprising:

a device that is normally nonconductive and that becomes conductive whensubjected to a potential approximately equal in level to saidpredetermined potential level;

series circuit means connecting said device across the potential inputto the supply;

signal amplification means comprising a transistor having emitter andbase leads included in said series circuit means and having a collectorlead out of which a shutdown current flows in response to current flowthrough said device; and

shutdown means within said supply for turning off said supply inresponse to said shutdown current.

15. A surge protection circuit for turning off a regulated power supplywhenthe potential input to the supply exceeds a predetermined potentiallevel, said circuit comprising: a device that is normally nonconductive,and that becomes conductive ,when subjected to a potential approximatelyequal in level to said predetermined level; series circuit meansconnecting said device across the potential input to said supply;amplification means for amplifying current flow through said device intoa shutdown current; and a constant current source comprising a referencepotential source having two terminals; a transistor having emitter,base, and collector electrodes, and having its base electrodes connectedto one terminal of said reference source, and having the constantcurrent flowing from its collector electrode; and a resistor connectingthe emitter electrode of said transistor to the remaining terminal ofsaid reference potential source, said shutdown current being fed intothe node formed by the junction of said re- 7 sistor with said emitterelectrode. 1 16. A regulated power supply for regulating the flow ofpower from a power source to a load, said supply comprising: a seriesregulating element;

predetermined potential and including said device;

shutdown means responsive to current flow through said device forproducing a current; and

a transistor connected to said reference potential source and to thecurrent generated by the shutdown means in such a manner that currentflow from the shutdown means causes the transistor to reduce themagnitude of the reference potential, whereby the power supply ceases topass current to the load when the potential input to the power sourceexceeds a predetermined potential level.

1. A regulated power supply of the type in which a reference potentialsource is used as a standard against which the output voltage or currentis compared, wherein the improvement comprises a surge protectioncircuit for turning off the supply when the potential input to thesupply exceeds a predetermined potential level including a device thatis normally nonconductive and that becomes conductive when subjected toa potential approximately equal in magnitude to said predeterminedlevel, series circuit means connecting said device across the potentialinput to the supply, and shutdown means connected to the referencepotential source for greatly reducing the magnitude of the referencepotential upon sensing a flow of current through said device.
 2. A surgeprotection circuit for turning off a regulated power supply when thepotential input to the supply exceeds a predetermined potential level,said circuit comprising: a device that is normally nonconductive, andthat becomes conductive when subjected to a potential approximatelyequal in level to said predetermined potential level; series circuitmeans connecting said device across the potential input to the supply;signal amplification means included in said series circuit means forproducing a shutdown current in response to current flow through saiddevice; and shutdown means within said supply for turning off saidsupply in response to said shutdown current.
 3. A surge protectioncircuit in accordance with claim 2 wherein the device is areverse-biased Zener diode having a Zener potential level approximatelyequal to said predetermined potential level.
 4. A surge protectioncircuit in accordance with claim 2 wherein the series circuit meanscomprises the amplification means and at least one resistive elementconnected in series with said device to form a series circuit, andwherein said series circuit is connected across the potential input tothe supply.
 5. A surge protection circuit in accordance with claim 2wherein the regulated power supply includes a potential reference devicethat is supplied with a current, and wherein the shutdown meanscomprises means for terminating this current whenever the shutdowncurrent is flowing.
 6. A surge protection circuit in accordance withclaim 2 wherein the regulated power supply includes a refeRencepotential, and wherein the shutdown means comprises means for greatlyreducing the magnitude of this reference potential.
 7. A surgeprotection circuit in accordance with claim 2 wherein the regulatedpower supply includes a series regulating element, and wherein theshutdown means comprises means for halting the flow of current throughthis series regulating element.
 8. A surge protection circuit forturning off a regulated power supply when the potential input to thesupply exceeds a predetermined potential level, said power supplyincluding a constant current source having a shutdown node, said circuitcomprising: a device that is normally nonconductive, and that becomesconductive when subjected to a potential approximately equal in level tosaid predetermined potential level; series circuit means connecting saiddevice across the potential input to said supply; and amplificationmeans for amplifying current flow through said device into a shutdowncurrent which is fed into the shutdown node within the constant currentsource.
 9. A surge protection circuit in accordance with claim 8 whereinthe constant current source within the supply feeds current to a Zenerdiode reference potential circuit within the supply.
 10. A surgeprotection circuit for generating a shutdown current in response to thepotential level of a potential source exceeding a predetermined maximum,said circuit comprising: a device that is normally nonconductive, andthat becomes conductive when subjected to a potential approximatelyequal in level to said predetermined level; and a series circuitconnected across said potential source, and including: said device, afirst current-limiting resistor connected directly to said device toform a series subcircuit, a transistor having emitter, base, andcollector electrodes, and having its base electrode connected to saidseries subcircuit, and a second current limiting resistor connected tothe emitter electrode of said transistor; whereby a shutdown currentflows from the collector electrode of said transistor when the potentiallevel of the potential source exceeds the predetermined maximum.
 11. Asurge protection circuit in accordance with claim 10 and furtherincluding a second Zener diode connecting the emitter electrode of saidtransistor to the end of said second current-limiting resistor that doesnot connect to the emitter of said transistor.
 12. A regulated powersupply for regulating the flow of power from a power source to a load,said supply comprising: a series regulating element; a first seriescircuit connected across said power source, and including said seriesregulating element and the load; a reference potential source; feedbackcircuit means for comparing the potential across said load with thepotential across said reference potential source, and for adjusting theconduction of said series regulating element to maintain the ratio ofthese two potentials substantially constant; a device that is normallynonconductive and that becomes conductive when subjected to a potentialwhich exceeds a predetermined potential level; a second series circuitconnected across said power source and including said device; andshutdown means responsive to current flow through said device forgreatly reducing the potential generated by the reference potentialsource, whereby the power supply ceases to pass current to the load whenthe potential of the power source exceeds a predetermined potentiallevel.
 13. A regulated power supply in accordance with claim 12 whichalso includes a constant source supplying a constant current to theregulating element, and wherein the shutdown means stops the flow ofcurrent from the constant current source.
 14. A surge protection circuitfor turning off a regulated power supply when the potential input to thesupply exceeds a predetermined potential level, said circuit comprising:a deviCe that is normally nonconductive and that becomes conductive whensubjected to a potential approximately equal in level to saidpredetermined potential level; series circuit means connecting saiddevice across the potential input to the supply; signal amplificationmeans comprising a transistor having emitter and base leads included insaid series circuit means and having a collector lead out of which ashutdown current flows in response to current flow through said device;and shutdown means within said supply for turning off said supply inresponse to said shutdown current.
 15. A surge protection circuit forturning off a regulated power supply when the potential input to thesupply exceeds a predetermined potential level, said circuit comprising:a device that is normally nonconductive, and that becomes conductivewhen subjected to a potential approximately equal in level to saidpredetermined level; series circuit means connecting said device acrossthe potential input to said supply; amplification means for amplifyingcurrent flow through said device into a shutdown current; and a constantcurrent source comprising a reference potential source having twoterminals; a transistor having emitter, base, and collector electrodes,and having its base electrodes connected to one terminal of saidreference source, and having the constant current flowing from itscollector electrode; and a resistor connecting the emitter electrode ofsaid transistor to the remaining terminal of said reference potentialsource, said shutdown current being fed into the node formed by thejunction of said resistor with said emitter electrode.
 16. A regulatedpower supply for regulating the flow of power from a power source to aload, said supply comprising: a series regulating element; a firstseries circuit connected across said power source and including saidseries regulating element and the load; a reference potential source;feedback circuit means for comparing the potential across said load withthe potential across said reference potential source and for adjustingthe conduction of said series regulating element to maintain the ratioof these two potentials substantially constant; a device that isnormally nonconductive and that becomes conductive when subjected to apotential which exceeds a predetermined potential level; a second seriescircuit connected across said power source and including said device;shutdown means responsive to current flow through said device forproducing a current; and a transistor connected to said referencepotential source and to the current generated by the shutdown means insuch a manner that current flow from the shutdown means causes thetransistor to reduce the magnitude of the reference potential, wherebythe power supply ceases to pass current to the load when the potentialinput to the power source exceeds a predetermined potential level.